Keira, who is being tipped for an Oscar nomination for her role as Cecilia Tallis, wore a silver-grey Rodarte dress, slashed across the front, and a tiara in her hair.

Directed by Joe Wright and adapted from the wartime novel by Ian McEwan, it is being hailed as one of the strongest British films in years.

It is in official competition for the Golden Lion award for best film at the Venice Film Festival and has been warmly received by critics.

The story tells of Cecilia Tallis, an upper class beauty whose fledgling love affair with the housekeeper’s son, played by James McAvoy, is destroyed by a tragic misunderstanding.

James also went over to greet his fans and sign autographs.

Keira said: "I think we are all proud of the whole film, mostly we are proud of Joe Wright, the director."

She said: "I find him an incredibly exciting director, and he’s given me two of the best characters I’ve played."

Asked if she had a preference for any particular kind of role she said: "No, I think the whole point is to change. I’m really lucky and I get the opportunity to change, and you can’t ask for much more than that as an actress."

Asked about her chances of winning an Oscar, she said: "The fact that the film is in the same sentence as the word Oscars means we’ve done a good job already. If it doesn’t get a nomination, it doesn’t take away from how proud I am."

Keira unveils film in Venice - Aug 30 2007
Keira Knightley has unveiled her new movie Atonement as she led the British charge at the Venice Film Festival.

The 22-year-old stars in the adaptation of Ian McEwan’s wartime novel and her performance is already being talked up as Oscar-worthy.

She plays Cecilia Tallis, an upper class beauty whose fledgling love affair with the housekeeper’s son (James McAvoy) is destroyed by a tragic misunderstanding.

Atonement has been chosen as the curtain-raiser for the Venice Film Festival and is being hailed as one of the strongest British films in years. It is in contention for the Golden Lion for best film.

Much has been made of Keira’s slender figure, which she showed off in a figure-hugging navy and white dress.

But the actress insisted that appearances can be deceptive.

"What this film shows is the danger when the line between fiction and fact gets blurred," she said.

"Magazine pictures, for example, are fiction. One person has done the make-up and hair, it’s somebody else’s clothes, it’s choreographed by someone else.

"In the media it’s very important to see different varieties of people - old, young, whatever size. As a woman myself I want to see variety. Hopefully that will happen more."

Keira played down her Oscar chances, saying: "As far as awards go, that’s neither here nor there. Atonement was a complete gift. I read the script and I cried. Any script that makes you cry is worth pursuing."

She said of her screen character: "I fell in love with Cecilia totally and utterly, partly because of the romance. I love a bit of romance."

Keira film to open Venice Festival - Juen 16 2007
Keira Knightley’s new movie Atonement has been chosen to open this year’s Venice Film Festival.

The adaptation of Ian McEwan’s novel, which also stars Vanessa Redgrave and James McAvoy, will be in contention for the Golden Lion.

It is only the second film from young director Joe Wright, 35, whose first was Pride and Prejudice.

"For the first time in its history, the opening film is the work of a young director," said festival boss Marco Muller.